The invention relates to a reflow soldering method and the apparatus thereof and more particularly relates to such method and the apparatus thereof wherein an air flowing fan is placed below a work transporting conveyer to draw or pull air into the apparatus from up to down toward a heater by means of a minus pressure produced by the fan so as to make constant the temperature of heated air, the flowing speed and volume thereof, and simultaneously to reduce the height of the apparatus as compared with a conventional reflow soldering apparatus. Further according to the invention, a mechanism for driving the fan is located at a specific place where there is little influence of air heating temperature so as to increase the endurance of the mechanism and to reduce the vibration and noise of the mechanism.
It is generally known that the reflow soldering apparatus requires no solder fusing tank, but is designed to progressively preheat a base board having electronic parts attached thereto by means of a paste like cream solder and transported to a heater by a conveyer and to heat the base board up to a soldering temperature of more than 230.degree. C. in a short time at the final process so as to fuse the cream solder to thereby solder the electronic parts to the base board.
According to the conventional reflow soldering apparatus, an electric heater is used to heat the base board by far region infrared rays radiated from the heater. In this case, it is required that the electric heater is set up to a temperature higher than a minimum temperature, that is, 150.degree. C.-250.degree. C. required to heat the the base board, because the heater heats up the base board spaced from the heater by nonmoving or stagnant air. The base board is therefore heated up to variable temperatures depending upon the speed of the conveyer transporting the base board relative to the heater. It is therefore necessary to control the speed of the conveyer in reference to a set temperature, up to which the base board is heated. In fact, it is difficult and troublesome to accomplish such speed and temperature control, because a most desired condition must be sought by repeated tests and checks each time a different type of base board is soldered.
In this connection, it is to be noted that the electronic parts mounted on the base board may each have a different heat capacity, and in dependence upon the heat capacity difference, there is a temperature difference of more than 50.degree. C. which may break the electronic parts of smaller heat capacity, the QFP (Quat-Flat Package), PLCC (Plastic Leaded Chip Carrier) and the like which are sensitive to heat. Moreover the rapid temperature rise in the preheating process will give a strong heat shock to the base board and the electronic parts mounted thereon.
The above mentioned defects and disadvantages may be eliminated by the so called vapor phase method in which a special organic solvent is vaporized up to an optimum temperature, for example, about 250.degree. C. which is maintained as the highest temperature which will not heat the base board up above the solder set temperature. Even this method may however destroy the base board and the electronic parts because the temperature of the vapor heat medium will rapidly rise in the preheating process of the base board.
Moreover the organic solvent used in the vapor phase method is very expensive and is consumed in the vaporization, and therefore the soldering cost is very high. Further in this vapor phase method the temperature of the base board may be raised easily, but the temperature of the heated base board after the soldering process is not easily dropped in the subsequent cooling process. This is because the vapor of the organic solvent attached to the base board remains at a temperature slightly lower than the soldering temperature and because the vapor has a heat capacity remarkably larger than that of the air. Further the organic solvent is poisonous so that care must be taken in its employment.
The applicant has developed a reflow soldering method and apparatus thereof employing a new air circulating system for eliminating the defects and disadvantages of the prior art as mentioned above. The inventions in connection with the mentioned method and the apparatus thereof have been filed in Japan as patent applications No. 62-12071 and No. 62-115456. However these inventions and the invention as seen in FIG. 11 showing a conventional reflow soldering apparatus 1, a plurality of air flowing fans 3 and fan driving motors 4 are additionally provided above a work transporting conveyer 2 resulting in increasing the height H of the apparatus, the height H being about 1.55m or more than that. Therefore if the operator 6 is less than 1.55m tall, his sight 6a is lower than the apparatus 1 and is blocked by the upper hood 1a of the apparatus 1, and he cannot see the objects beyond the apparatus in the factory. Actually an operator of taller than 1.75m can see the objects beyond the apparatus, but cannot see the objects of less than 1.55m tall located or existing beyond the apparatus. This will detract from the supervisory control of the factory.
Further according to the conventional apparatus, the fans 3, the electric motors 4 and other mechanisms in connection therewith are arranged above heaters 8. These members are therefore all exposed to a high temperature of the air coming up as the air is heated by the heater. This will shorten the life of these members. Moreover these members located at the upper portion of the apparatus will give rise to vibrations and noises.
Further according to the conventional apparatus, the fans 3 located above the heaters 8 will be driven to push the air against the heaters by means of an increased pressure. This will however fail to make the air flow constant over the faces of the heaters resulting in variable temperature distribution of the heated air.